Sensing systems often benefit from the fusion of dissimilar signal types and there are numerous advantages to integrating different modality measurements, such as radio frequency and optical/infrared, into one platform. The information derived from such measurements is complimentary and may provide advantages under certain operating conditions. Radars, in the basic forms, are used for ranging and Doppler shift detection. Synthetic Aperture Radar additionally allows a system to generate an image based on the synthetic aperture resulting from the motion of the sensor or an object. Radars can provide high precision distance and velocity measurements suitable for automatic target tracking, while optical or infrared sensors provide superior results with respect to angular resolution of the signal. Also, radars receive much stronger returns from metal and other conductive objects, such as cables and equipment chassis, and could provide views that penetrate underground and through canopies. Likewise, the optical and infrared data has the benefit of being relatively simple to interpret.
Some targets that are designed to channel away RF radiation via geometry and stealth skin material may be more easily detected in the optical/infrared (O/IR) image than by using a radar based system. O/IR images are easier to decipher in high RF saturated environments, and it usually provides much more precise angular resolution in cluttered or dense urban environments. Also, in the absence of supplemental data, jamming and spoofing activity could substantially reduce the reliability of the RF based systems. In many cases, the RF works better for detection, while O/IR signature works better for recognition. It is also noteworthy that O/IR information will be less effective when subjected to high water vapor content in the atmosphere or other environmental conditions.
Many difficulties related to RF information collection in a high clutter environment are caused by multiple RF sources or/and multi-path scattering. The present challenges in spectrum allocations for active radar systems, and sensitivity limitations of passive RF systems, could substantially limit the reliability of the RF data. Stealth targets produce much lower return due to “skin” material design, while some radar targets are designed to channel the RF scattering away from the receiver or to generate decoy signals. For example, a missile or an aircraft could release a decoy cloud. In the presence of countermeasures, radar could lock onto the elements in the decoy cloud or the target. Ideally however, a sensor should discriminate between the target and decoys by using complimentary information that is coming from different modalities.
In most cases, measurements in different modalities from discrete, spatially separated sensors substantially increase the complexity of the information exchange while producing more data for human analysis. By way of example, collecting separate data tracks increases the complexity of the decision system. There are other numerous advantages of collocated multi-modal sensors including elimination of the registration procedure, or alignment to a common datum, for the multimodal data.
In order to direct and concentrate RF electromagnetic energy to detectors, various antennas (horns, resonators, etc.) are used. To concentrate the optical or IR energy on detectors, lenses, curvilinear mirrors, etc. are used. In many cases, the materials used for each such purposes have completely different effects when subjected to other part of the spectrum. For example, metal or magnetic material based antennas are usually attenuate or obscure the optics/IR signal. Conversely, many optical/IR concentrators are made from bulk dielectric materials that affect propagation of RF waves and change their properties with variations of wavelength. In many cases the optical components must be rotated toward the direction of interest using gimbal that is further complicate RF propagation patterns. Additionally, dielectric materials that are in use for optical and infrared sensors also could undesirably affect the RF phased array performance. The integration with antennas in radio wave ranges is often complicated due to requirements to generate a radiation pattern having a footprint in an order of magnitude of at least a quarter of the operative wavelength. For many applications, that could be many orders of magnitude larger than optical/RF waves.
According to the superposition principle, electromagnetic fields in free space result from the vector sum of the individual components. For this purpose, the combination of different modalities could be implemented by materials with the same values of propagation parameters for different wavelengths or using only reflective systems which allow formation of the image for any wavelength in free space without distortions and chromatic aberrations. Unfortunately, a reflection-based omnidirectional focusing apparatus configured to receive wide multi-mode (or multi-physics) signals is not known to the prior art. As a result, there exists a need for an omnidirectional dual RF and Optical sensor implemented as a reflective focusing aperture.